Yanhong Tian

One-Step Fabrication of Stretchable Copper Nanowire Conductors by a Fast Photonic Sintering Technique and Its Application in Wearable Devices.

Copper nanowire (CuNW) conductors have been considered to have a promising perspective in the area of stretchable electronics due to the low price and high conductivity. However, the fabrication of CuNW conductors suffers from harsh conditions, such as high temperature, reducing atmosphere, and time-consuming transfer step. Here, a simple and rapid one-step photonic sintering technique was developed to fabricate stretchable CuNW conductors on polyurethane (PU) at room temperature in air environment. It was observed that CuNWs were instantaneously deoxidized, welded and simultaneously embedded into the soft surface of PU through the one-step photonic sintering technique, after which highly conductive network and strong adhesion between CuNWs and PU substrates were achieved. The CuNW/PU conductor with sheet resistance of 22.1 Ohm/sq and transmittance of 78% was achieved by the one-step photonic sintering technique within only 20 μs in air. Besides, the CuNW/PU conductor could remain a low sheet resistance even after 1000 cycles of stretching/releasing under 10% strain. Two flexible electronic devices, wearable sensor and glove-shaped heater, were fabricated using the stretchable CuNW/PU conductor, demonstrating that our CuNW/PU conductor could be integrated into various wearable electronic devices for applications in food, clothes, and medical supplies fields.

Thermomechanical behaviour of PBGA package during laser and hot air reflow soldering

In this paper, the temperature distribution in laser reflowed solder balls and warpage of the package upon temperature change were simulated using a finite element method, and effects of different laser heating means on the temperature distribution have been discussed in detail. The results of simulation show that solder balls reflowed by laser heating power would not damage the silicon chip and the package and that with laser, the advantages of a short heating time, a lower temperature in the package and smaller deformation were obtained as compared with a traditional heating method such as the hot air reflow method. Also, experiments on a plastic ball-grid-array (PBGA) solder ball laser reflow were carried out. Results show that the surface of the solder bumps obtained by the laser reflow method with proper parameters is much smoother than that obtained by the hot air reflow method.

Effect of solidification on solder bump formation in solder jet process: Simulation and experiment

To investigate the influence of the solidification on the solder bump formation in the solder jet process, the volume of fluid (VOF) models of the solder droplets impinging onto the fluxed and non-fluxed substrates were presented. The high speed camera was used to record the solder impingement and examine the validity of the model. The results show that the complete rebound occurs during the process of the solder droplet impinging onto the fluxed substrate, whereas a cone-shaped solder bump forms during the process of the solder droplet impinging onto the non-fluxed substrate. Moreover, the solder solidification results in the lift-up of the splat periphery and the reduction in the maximum spread factor.

Bonding mechanism of ultrasonic wedge bonding of copper wire on Au/Ni/Cu substrate

Abstract The ultrasonic wedge bonding with d 25 μm copper wire was achieved on Au/Ni plated Cu substrate at ambient temperature. Ultrasonic wedge bonding mechanism was investigated by using SEM/EDX, pull test, shear test and microhardness test. The results show that the thinning of the Au layer occurs directly below the center of the bonding tool with the bonding power increasing. The interdiffusion between copper wire and Au metallization during the wedge bonding is assumed negligible, and the wedge bonding is achieved by wear action induced by ultrasonic vibration. The ultrasonic power contributes to enhance the deformation of copper wire due to ultrasonic softening effect which is then followed by the strain hardening of the copper wedge bonding.

Effect of Au-Sn IMCs’ formation and morphologies on shear properties of laser reflowed micro-solder joints

Purpose – The purpose of this paper is to investigate the effect of typical morphologies of Au-Sn IMCs (intermetallic compounds) at the interfaces of solder and pads on shear properties of laser reflowed micro-solder joints. Design/methodology/approach – Sn-2.0Ag-0.75Cu-3.0Bi (SnAgCuBi) solder balls (120 μm in diameter), pads with 0.1, 0.5, 0.9 or 4.0 μm thickness of Au surface finish, and different laser input energies were utilized to fabricate micro-solder joints with Au-Sn IMCs having different typical morphologies. The joints were performed by a shear test through a DAGE bond test system. Fracture surfaces of the joints were analyzed by scanning electron microscopy and energy-dispersive X-ray spectrometry to identify fracture modes and locations. Findings – Morphologies of Au-Sn IMCs would affect shear properties of the joints remarkably. When needle-like AuSn4 IMCs formed at the interfaces of solder and pads, almost entire surfaces presented the manner of ductile fracture. Moreover, shear forces of ...

Effect of intermetallic compounds on fracture behaviors of Sn3.0Ag0.5Cu lead-free solder joints during in situ tensile test

In this paper, in situ tensile tests under various amounts of deformation were performed on Sn3.0Ag0.5Cu lead-free solder joints subjected to multi-reflow and isothermal aging processes by using a scanning electron microscope. Microstructure evolution and deformation behavior of the solder joints were observed. Effects of the intermetallic compound (IMC) Cu6Sn5 on fracture behaviors of the solder joints were investigated. Results showed that the Sn3.0Ag0.5Cu lead-free solder joints contained only a few Sn grains, and the sequence and degree of plastic deformation varied for the different grains in the same solder joint due to the strong anisotropic properties of Sn grains. Further experiments revealed that plastic deformation occured primarily in the form of slip bands in the solder joints during the in situ tensile test. Various fracture modes including intergranular and phase boundary fractures were observed. The fracture behaviors of solder joints were significantly affected by morphologies and distributions of the Cu6Sn5 IMCs. It was found that Cu6Sn5 particles located at the grain boundaries are apt to become crack sources, and that the long rod shaped Cu6Sn5 were easily broken. However, spherical Cu6Sn5 hardly deformed during the tensile test, resulting in dynamic recrystallization. In this case, fracture occured at the sub-grain boundaries.

Room-temperature direct bonding of silicon and quartz glass wafers

We demonstrate a facile bonding method for combining Si/Si, Si/quartz, and quartz/quartz wafers at room temperature (∼25 °C) using a one-step O2/CF4/H2O plasma treatment. The bonding strengths were significantly improved by adding a small amount of CF4 into the oxygen plasma, such that reliable and tight bonding was obtained after storage in ambient air for 24 h, even without employing heat. Moreover, by introducing water vapor during O2/CF4 plasma treatment, uniform wafer bonding was spontaneously achieved without applying an external force. The fluorinated surface asperities appear to be softened more easily by the interfacial water stress corrosion, enabling reliable bonding at room temperature. Additionally, adding an optimized amount of water vapor to the O2/CF4 plasma increases sufficiently the amount of hydroxyl groups without eliminating the CF4 effect. The additional water adsorbed on the surface may help to close the gap between the bonded wafers, resulting in better bonding efficiency.

Coupling effects of mechanical vibrations and thermal cycling on reliability of CCGA solder joints

Abstract The microstructures and crack propagation behavior of CCGA (ceramic column grid array) solder joints after sinusoidal vibration loading, random vibration loading, and thermal cycling test have been discussed in this study. The failure mechanism of solder joints was analyzed using an experimental method and finite element analysis. It was found that the failed solder joints mainly distributed at the peripheral area in the solder column arrays and the crack initiation was mainly caused by mechanical vibrations. The deformation of PCB (printed circuit board) introduced by mechanical vibrations brought the outermost solder columns in CCGA devices with significant stress concentration and induced the initiation of cracks. Furthermore, cracks propagated during the process of mechanical vibrations and thermal cycling. The cracks propagated rapidly and the solder joints finally failed. The structure of the PCB holder was improved to relieve the vibration response from the peripheral joints. No visible crack was found in the solder joints after the same mechanical vibrations and thermal cycling test. The reliability of solder joints have been greatly improved with the new PCB holder.

Molecular dynamics simulation of joining process of Ag-Au nanowires and mechanical properties of the hybrid nanojoint

The nanojoining process of Ag-Au hybrid nanowires at 800K was comprehensively studied by virtue of molecular dynamics (MD) simulation. Three kinds of configurations including end-to-end, T-like and X-like were built in the simulation aiming to understand the nanojoining mechanism. The detailed dynamic evolution of atoms, crystal structure transformation and defects development during the nanojoining processes were performed. The results indicate that there are two stages in the nanojoining process of Ag-Au nanowires which are atom diffusion and new bonds formation. Temperature is a key parameter affecting both stages ascribed to the energy supply and the optimum temperature for Ag-Au nanojoint with diameter of 4.08 nm has been discussed. The mechanical properties of the nanojoint were examined with simulation of tensile test on the end-to-end joint. It was revealed that the nanojoint was strong enough to resist fracture at the joining area.

Mechanism of low temperature Cu-In Solid-Liquid Interdiffusion bonding in 3D package

Three-dimensional (3D) integrated circuit (IC) packaging technology is under rapid development to realize high-density and high-speed transmission, and through silicon via (TSV) as advanced bonding technology is used to connect the wafers in chips extensively. Cu-In Solid-Liquid Interdiffusion (SOLID) low temperature bonding is a promising process for TSV interconnection. In this paper, the joint consisting of complete intermetallic compounds (IMCs) was designed as a potential alternative method to improve the reliability of solder joints in the electronic assemblies and systems operated at elevated temperatures. The samples of Si/Ti/Cu/In structure were used for the SOLID bonding. Scanning electron microscope (SEM) and Energy-dispersive X-ray (EDX) were used to observe the interfacial microstructure which showed the joints all consisted of the complete Cu-In IMCs. The appropriate bonding temperature and peak temperature were determined according to the result of observation. The results showed that at the temperature of 260°C, the Cu11In9 and Cu2In phase will be firstly formed in the solder. The Cu2In and Cu7In3 phase will be firstly formed at the temperature of 360°C.